Fuel cells have advantages that they have high energy efficiency since electric energy is electrochemically taken out directly from a fuel, and further they are easily in harmony with surroundings since materials discharged therefrom are mainly made of water. For this reason, made are attempts of applying the cells to automobiles, dispersed power sources, information electronic devices, and so on. In particular, in connection of information electronic devices, attention is paid thereto as power sources which can be driven for a long term instead of lithium cells, and various information electronic devices each mounting a fuel cell are suggested.
Out of fuel cells wherein methanol is used as methanol, the so-called direct methanol fuel cell (referred to as DMFC hereinafter), wherein liquid methanol is directly oxidized to take out electricity, has an advantage that a cell system therefor can be relatively easily made up since a reforming unit and so on are unnecessary.
The power generating principle of the DMFC is represented by the following expressions (1) to (3):Fuel element reaction: CH3OH+H2O→6H++6e−+CO2  (1)Air electrode reaction: 6H++6e−+1.5O2→3H2O  (2)Total reaction: CH3OH+1.5O2→2H2O+CO2  (3)
As represented by the expression (2) or (3), water is generated in the air electrode in the DMFC. Water is in the form of water vapor when it is generated. However, in accordance with the structure or the material of the air electrode and conditions for the treatment, the water partially turns into dew condensation water. The dew condensation water is partially discharged outside the air electrode while the other part of the water remains in the air electrode. This causes an increase in the wettability of the gas diffusion electrode or the cathode catalyst layer with the passage of time. Accordingly, the DMFC has a problem that pores which are channels for supplying oxygen gas are blocked up in the gas diffusion electrode and the cathode catalyst layer. The performance of the air electrode depends on the supplied oxygen amount; therefore, when the pores are blocked up, oxygen gas is not sufficiently supplied to the electrode to result in a phenomenon that the performance falls. When the cell is stopped for a long term, the polymeric electrolyte or electrolyte membrane in the electrodes is not swelled by the generated water sufficiently. Thus, the amount of a network based on hydrogen ions decreases so that the ion conductivity of the electrodes falls, resulting in a drop in the cell performance.
As a technique for collecting water generated in the cathode, JP-A-2006-49153 is known.